Recently, the use of flat display panels has increased, a liquid crystal display (LCD) being a representative flat display panel.
Generally, unlike an existing cathode ray tube (CRT), such an LCD requires a backlight module providing light uniformly to an entire screen.
Specifically, such a backlight unit has a configuration by which light is uniformly provided to a rear surface of the LCD, and is configured such that a light source, i.e. a light-emitting diode (LED), is disposed on an edge of a light guide plate, and the light guide plate includes a reflective plate on a lower surface thereof to transfer light generated by the light source upwardly.
In the aforementioned configuration, the light generated by the light source is transferred upwardly by the light guide plate and the reflective plate, and the transferred light is transferred upwardly through a light condensing sheet uniformly.
In addition thereto, a separate reflective polarization sheet is provided on an upper portion of the light condensing sheet to only transmit specific polarized light upwardly, thereby stably transferring condensed light externally.
A general prism sheet may be used as the light condensing sheet.
As described above, the backlight unit is configured such that the light generated by the light source provided on the one side surface is transferred upwardly by the light guide plate and the reflective plate and the transferred light is uniformly condensed through the light condensing sheet.
Meanwhile, when the backlight unit includes a reflective polarization sheet, light can be transferred upwardly in a state of being uniformly condensed. Thus, the reflective polarization sheet is widely used. However, the reflective polarization sheet may be bent due to an increase in temperature, caused by elongated usage of the backlight unit.
Generally, the reflective polarization sheet is formed by hardening a multilayer stack in a state of being expanded, and is contracted when a temperature thereof is increased. Cooling and hardening are performed using a separate cooling roll at the time of forming the reflective polarization sheet. One surface of the reflective polarization sheet, contacting the cooling roll, has a different contraction ratio to that of the other surface hereof, not contacting the cooling roll, due to a difference in cooling rates.
Therefore, when a temperature of the reflective polarization sheet is increased, due to a contraction ratio difference, bending may be generated, such that one surface of the reflective polarization sheet may protrude.
In this regard, when the temperature of the reflective polarization sheet is not increased, the reflective polarization sheet is not bent. However, when the reflective polarization sheet is used in a general LCD, heat is generated in the general LCD. Thus, the temperature of the reflective polarization sheet may be increased, contracting the reflective polarization sheet.
In this case, the reflective polarization sheet may be bent. Thus, light transferred through a lower portion of the reflective polarization sheet may not be uniformly transferred externally.
In addition, when an LCD is continuously used in a state in which the reflective polarization sheet is bent, exfoliation may be generated due to humidity or heat.